1. Field of the Invention
This invention relates to a pad for handling force loads to prevent wear on the load bearing surface of a pedestal of a rail vehicle, and more particularly relates to improvements in force handling in an arrangement where a wear plate specifically designed for convenient and removable installation in the pedestal opening of a railway side frame is used in order to prevent wear on the load bearing surface of the pedestal.
2. Brief Description of the Related Art
For the sake of brevity, the teachings and complete disclosures of previous U.S. Pat. Nos. 3,897,736 and 4,203,371 are incorporated herein by reference. In many cases, a bearing adapter is provided over the axle bearing, said adapter having a slightly arcuate top surface which bears directly against a corresponding downwardly facing surface in the pedestal opening of the side frame. This arcuate top surface provides the wheel and axle with a freely pivoting end condition to avoid binding loads on the roller bearing. In order to reduce wear on the pedestal, a wear plate is placed between the downwardly facing pedestal surface and the top surface of the bearing adapter. The wear plate may have side lips and may clip onto the pedestal.
In service, movement or frictional sliding may occur between the bearing adapter and the clip-on pedestal wear plate, which may cause damage to the clip-on pedestal wear plate, resulting in the need to replace the wear plate. This condition also may result in the loss of the freely pivoting end condition and may weaken the frame at a load-bearing location. Repair of the frame surface is both expensive and time-consuming, since the worn surface must be ground down to return it to a flat condition. The amount of grinding allowed, however, is limited by structural considerations; after the limit has been reached, the side frame casting is condemned. Replacing the clip-on wear plate is less expensive, but still costly, as it involves removing the railroad car from service and jacking the car so that the broken clip-on wear plates can be removed and replaced.
The plate working load and stress are defined as the frictional forces applied to the wear plate by the bearing adapter as a rail car in service shifts and moves about laterally. These frictional forces induce a tendency for lateral movement of the wear plate, and are opposed by the corresponding friction developed between the plate top and the pedestal roof surface. If this opposing frictional force is insufficient to resist this movement, additional bending load and stress are imposed on the side lips of the wear plate. The sensitivity to imbalance in these frictional forces, and hence the tendency to impose stress and load on the plate side lips, increases with heavily loaded rail cars such as coal cars.
The result of the total installation related tensile stresses in combination with stress related to bearing adapter friction can result in a significant shortening of the plate service life. In some cases, the total tensile stresses developed may reach the yield strength of the plate and thereby cause bending of the plate. In other more severe cases the ultimate strength of the plate may be reached causing cracking of the plate.
The problem of the frictional force load handling has been further complicated as of late because of recent frame painting practices. As environmental concerns have caused an effort to reduce volatile organic emissions from sources including paints, rail car frames are increasingly being painted with solvent-free and alternative solvent based paints. One of the disadvantages of these paints is that a resultant painted surface will have a significantly lower coefficient of friction as compared to a surface painted by older “traditional” paints; at times the new paint may even be thought of as acting as a sort of lubricant. This has the disadvantageous result of greatly reducing the frictional force between the plate top surface and the pedestal roof surface, thereby increasing the effective tensile stress in the plate. This has in turn resulted in an increased occurrence in wear plate bending and cracking.
Increasing the thickness of the plate would seem to offer a means to achieving increased plate strength sufficient to resist lateral movement and consequent failure. The benefits of increasing plate thickness, however, are limited. A practical limit on plate thickness exists as installation bending stresses caused as the side lips are forced apart during plate installation increase in direct proportion to the plate thickness. The difference between these installation bending stresses and the ultimate stress at which failure occurs determine the working capacity of the plate to resist movement. At some thickness a maximum plate working capacity is reached and further thickness increases actually decrease working capacity.
For the above stated reasons, an unresolved need exists for a pedestal wear plate or wear plate assembly with an improved ability to withstand tensile stresses and thereby enjoy a reduced occurrence of bending and cracking.